Avian song learning is a powerful model for studying how experience influences neural and behavioral development. During song learning, birds memorize conspecific songs and use auditory feedback to shape their vocalizations to these stored models. This song learning often is restricted to species-specific sensitive periods, and this proposal is aimed at identifying neural events that encourage and constrain this learning. The studies focus upon two neural events that accompany song learning: developmental changes in the density and pharmacology of the N-methyl-D-aspartate (NMDA) glutamate receptor subtype, and a massive addition of new circuitry within the vocal motor pathway. NMDA receptors have been linked to many instances of experience-dependent neural plasticity and their activation is necessary for the memorization phase of song learning. Behavioral pharmacology and Golgi staining will be combined in zebra finches to determine whether NMDA receptor activation prompts the decline in dendritic spines that occurs in one song region during learning. Also, receptor autoradiography will be used to determine 1) if auditory manipulations that delay closure of the sensitive song learning period similarly extend developmental changes in NMDA receptor density or pharmacology, 2) if hormonal manipulations that affect song development alter the developmental profile of NMDA receptor expression, and 3) if recurring periods of vocal plasticity in adult canaries are accompanied by changes in NMDA receptor density or pharmacology. The second set of experiments will explore further the hypothesis that neuronal incorporation within the vocal motor pathway regulates behavioral plasticity. Retrograde tracing will be used in zebra finches to identify cell types contributing to a provocative correlation between the number of song-related neurons and the amount of song material learned. Also, measurements and manipulations of yolk testosterone levels will be used to assess if variations in prehatch hormone levels relate to individual differences in neuron number or learning capacity. Finally, adult Bengalese and zebra finches will be compared using thymidine autoradiography and retrograde tracing to determine if species differences in HVC neuron addition may account for species differences in vocal deterioration after deafening. Many behaviors (e.g., language acquisition, social attachment, imprinting) exhibit developmental changes in their sensitivity to environmental stimuli, and these studies will help define neural mechanisms that encourage learning and constrain periods of unique susceptibility.